The interaction of biologically active peptides with macromolecules will be studied using primarily 13C nuclear magnetic resonance (NMR) techniques. 19F NMR will also be used. The neurohypophyseal hormones, oxytocin and arginine vasopressin (AVP) synthesized (via solid phase techniques) specifically enriched with 13C in various positions, or with neurophysins (NP). The latter are a group of proteins which bind the hormones in vivo. About a dozen positions in the hormone will be monitored. It should be possible to deduce a precise three dimensional structure of the hormone when it is bound to NP. Various experiments directed toward this end will be performed. 1) The protein will be specifically modified so as to create strong metal binding sites, while not affecting hormone binding. By using paramagnetic metals such as gadolinium and observing the effects on the relaxation times of the enriched 13C or 19F atoms of the hormones bound to NP, distance measurements will be determined. 2) A theoretical analysis of all chemical shifts observed in the bound hormone will be undertaken. 3) Relaxation time measurement for both 13C and 19F will be performed. 4) Several 1H-13C 3-bond coupling constants for the bound hormone will be measured. This will give information on side chain rotamer populations. These studies will be further aided by the use of specifically labeled D-amino acid diastereoisomers of oxytocin (and AVP) which interact with NP, but with binding constants differing from those of the native hormones. The experiments will yield vaulable information on the storage and release of oxytocin and AVP from neurosecretory granules, as well as demonstrating new methods for studying protein-hormone interaction.